GB2147614A - Heat transfer printing - Google Patents

Abstract

The invention provides a structureless medium which is 'transparent' to sublimable dye during the heat transfer printing process. The structureless, dye-transparent medium may be employed in two ways. It may be used to carry a design which is impermeable to sublimable dye so that a reserved print of the decorative design may be obtained on a suitable dye-receptive substrate of thermoplastic fabric or film by interposing the medium plus the impermeable decorative design between a sheet of heat transfer material and the substrate during heat transfer printing. Since the plastic film medium plus the impermeable decorative design (which are preferably adhesively combined) are not damaged during the heat transfer printing process, they may be re-used any number of times to impose the same design on further samples of the substrate using new transfer material at each printing. Alternatively, the structureless, dye-transparent medium may be interposed between a sheet of heat transfer material and a dye-receptive substrate when it is desired to avoid contamination of the substrate by materials other than the sublimable dye (such as adhesives, resins, binders, etc) which may be present on the sheet of transfer material.

Description

SPECIFICATION Heat transfer printing This invention relates to heat transfer printing using sublimable dyes. The invention provides a structureless medium which is 'transparent' to su blimable dye during the heattransfer process. The structureless, dye-transparent medium may be employed in two ways. It may be used to carry a design which is impermeable to sublimable dye so that a reserved printofthe decorative design can be obtained on a suitable dye-receptive substrate ofthermoplastic fabric orfilm by interposing the medium plus the impermeable decorative design between a sheet of heat transfer material and the substrate during heat transfer printing.Alternatively, the structureless, dyetransparent medium may be interposed between a sheet of heat transfer material and a dye-receptive substrate when it is desired to avoid contamination of the substrate by materials otherthan the sublimable dye (such as adhesives, resins, etc) which may be present on the sheet oftransfer material.

It is already known to produce decorated fabrics and plastics by exposing them to a sheet oftransfer material, such as a smooth sheet of paper carrying a vapourisable or sublimable dyestuff. By the application of heat to the transfer sheet when pressed against the fabric or plastic, the dyestuff is caused to migrate to thefabric or plastic in vapourform andthere is produced on the fabric or plastic a reverse replica of the decorative design in which the dyestuff was laid down on the transfer sheet. During this heat transfer process, a substantial proportion ofthe dyestuff carried on the transfer sheet is removed by the migration process and the decorative design may thus onlybe printed once on the substrate at its full required strength.This is not a disadvantage if many thousands of articles of fabric or plastic are required to be decorated because the costs of preparing the artworkforthe decorative design and the costs of preparing the sheets oftransfer material on expensive equipment are amortised over manythousands of identical sheets of transfer material. If only one of two articles offabric or plastic are to be decorated, then this also is inexpensive as the sheets of transfer material may be prepared by a number of means, for example, by the assembly of non-transferably marked sub-units ofthe decorative design as in my copending Application Number 8,323,668 or by hand drawing or painting with special caryons, paints orfelt-tipped markers containing sublimable dyes or bythe use of an electrophotographic process as described in British Patent Specification 1,497,457.Unfortunately, however, in the case of assembling non-transferably marked sub-units or hand drawing or painting, it is expensive andtime-consumingto prepare morethanasmall number of identical designs. In fact, with hand drawing or painting it is virtually impossible to prepare even two identical designs because of variationsthatarise naturally in the depth of colour laid down each time bythe artist or by slightvariations in the width and placing of brush or carton lines.In the case ofthe electrophotographical ly prepared sheets oftransfer material, although this process is suitable for preparing between, say, two and a hundred sheets oftransfer material from one piece of artwork, unfortunately the cost of the equipment is high (leading to a high cost per sheet oftransfer material) and the quality of the final transferred design is not satisfactory for critical applications. In particular,the electrophotographic process cannot be set up so that there is a complete absence of sublimable dye in the areas ofthe sheet oftransfer material where no decoration is intended to be present.There is always present a small amount of sublimate dye in the form of speckles in whatshould be clearareas of the sheet of transfer material (especially near the edges of any part ofthe decorative design) and during the heat transfer process these speckles of dye produce coloured or blackspeckles (according tothe dye colour) in the background of the decorative design on the substrate.

These speckles are aesthetically unattractive and whilsttheymaysometimes be scraped off the sheet of transfer material before it is heated in contact with the substrate, this cannot be done with complicated or intricate decorative designs because ofthe danger of damaging wanted parts of the sheet of transfer material. It is also known in the art of heat transfer printing to decorate suitable substrates by heat transfer printing through a porous material which may or may not have an affinityforthe sublimate dye and which mayor may not be permeable to the dye vapour.Such processes are referred to in United States Patent 4,049,374. In this publication an embossing sheet having no affinity or retention property for sublimable dyes is interposed between the transfer sheetandthesubstrate during the application of heat and pressure. This embossing sheet is made of a porous material such as fibreglass which can let through the major part ofthe subliming dyestuff as vapour. With this process there is obtained on the fabric or plastic an embossing orsurfacetexturing effect practically without any change in the design transferred to thefabric or plastic.However, there is some change in the the design because the embossing sheet is not uniformly permeableto the vapourof the sublimable dyestuff and, in addition, by its textured nature,the embossing sheetcannot be in such intimate contact with the substrate that some of the dyevapourcannot be prevented from 'wandering' in the interstices ofthe embossing sheet and thereby blurring the definition of fine lines or colour boundaries forming part of the decorative design.Another process (also referred to in United States Patent 4,049,374) employing an embossing sheet is one in which the embossing sheet is substantially or largely impermeableto the dyestuffvapouror has an affinity for the dyestuff so that the dyestuff reaches the fabric or plasticonlythrough defined perforations in the embossing sheet to define the embossing pattern therein and hence colours onlythe portionsofthe fabric or plastic opposite the perforations. In this process,thedyestuffappears on the fabricorplastic in a pattern which is a subtractive combination of the pattern which is present on the transfer sheet with the perforation pattern on the embossing sheet.Usually the embossing sheet (for example, a lace, crochet work or other openworktype offabric) is soiled with thedyestuffduringthetransferoperation and can be used once only, unless a cleaning operation is performed on it; of course,the fabric or plastic is also embossed with the pattern ofthe embossing sheet.

I have nowfound a way oftransfer printing designs on to fabrics and plastics in which a design which is reserved in some suitable impermeable substance is supported on a plasticfilm which is permeable atthe molecular level to sublimable dyestuffs whereby such a combination may be used time aftertimetotransfer the design to the fabric or plastic without embossing the fabric or plastic during thetransfer printing process, without any hint ofthe structure of the supporting film appearing in the transferred design and without any need forcleaning the plasticfilm, unlike the effects observed with previous permeable embossing sheets.I have also found thatthis invention may be used in the transfer of designs to fabrics and plastics where it is desirable or essential that materials such as adhesives, resins, carriers, etc present on the sheet oftransfer material carrying the design in sublimable dyestuff do not contaminate the fabric or plastic.

According to the present invention there is provided a means of heat transfer printing in which sublimable dye is transferred to a receiving substrate by moleculardiffusionthrougha removableplastic5'm having an affinity forthe sublimable dye.

Thus, in particular, the decorations which it is desired to transfer print upon substrates of fabric or plastic may be any of an almost infinite range of designs. For example, they may be names or slogans together with photographs, sketches or caricatures of artists, singers, politicians or other well known figures. They may be geometrical or abstact designs.

Such designs may be required to be imposed upon T-shirts, sweaters, caps, blouses, dresses or other garments or textiles. It may be desired to impose the design in morethan one place on the article in which case it may be essential that the designs in all places are identical. Designs may be be in a single colouror in multiple colours. Other possibilities are decorative or descriptive labels in shops or stores ortransparent or opaque panels for use in displays and exhibitions or murals or decorative surfaces in or on buildings or vehicles, etc. Many possibilities suggest themselves oncethe basic invention is disclosed. Since the decoration is only transferred to the substrate of fabric or plastic when the article is required, the heattransfer printing process eliminatesthe costly holding of stocks of ready printed articles.However, with the conventional transfer printing process, it is still necessaryto hold stocks of sheets oftransfer material on which the designs are already carried. However, with the process ofthe present invention (when it is employed for carrying impermeable decorative designs), the number of designs of sheets of transfer material required to be held in stock is very much reduced because the decorations are partially or wholly generated from the impermeable designs carried on the structureless, dye-transparent medium.

The heattransfer process will work satisfactorily with a number of materials which are, generally speaking, thermoplastics. Thus the substrates to be decorated may be composed of a textile material, a nonwoven article, paper or any surface (such as metal, plastic, wood, etc) impregnated, coated or covered with or consisting of an acrylic polymer, polyamide, polyester, epoxy resin, vinyl resin or polyurethan.

Preferred substrates are textiles containing 50% or more ofsurfacefibres of polyester or poiyamide or a sheet of plastic material or plastic coated material where the plastic is a linear or cross-linked polyester orpolyamide. Polyethyleneterephthalateornylor6:6 sheet with oriented molecules, optionally containing a white or coloured pigment are materials even moreto be preferred. Such plastic sheets may be adhered to a support of paper, plastic, metal, etc or they may be coated with athin metallic film orothercoating on the opposite face to thatwhich is to be decorated.The heat transfer process is advantageously carried out by heating the sheet oftransfer material in contact with the substrate offabric or plastic at a temperature above 140 C and preferably above 170"C. Obviously the preferenceforthistemperature imposes limits own the thermoplastics which may be employed since they must be chemically and physicallystableatthe transfertemperature. The heating operation is preferably carried outfor between 10 and 60 seconds and more preferably between 15 and 45 seconds.

Transfer sheets are normally prepared by conventional processes used for printing orforming designs on paper or similar surfaces, for example, by hand drawing or painting, by printing by any of the known means such asflexography, gravure, letter press, screen printing, lithography, etc or by any other meanswhich leads to a sharp and distinct image free from speckle or other extraneous transferable item.

Many materials such as paper or metal foil such as aluminium foil maybe usedforthetransfersheets. For the sake of good receptivity to the ink, low cost and ease ofsupply, paper is preferred and a lightweight, chemically pure grade manufactured especially as a transfer printing article is a suitable product since many ranges of suitable inks are available for use with and are expecially compatible with such papers. The decorative design may be printed as it is required to appearon the substrate of fabric or plastic or it may need to be printed as a mirror image of its final appearance. If decorative design is to be viewed through the substrate (ie if the substrate is transparent plastic film or sheet) then the decorative design does not require two be present as a mirror image.If, however, the decorative design is to be viewed on the surface ofthe article, then it must be in mirror image form. Inks containing vapourisable dyes are readily available in ranges suitable for application by any of the techniques mentioned aboveforthe imposition of the decorative design on the transfer sheet. It is desirable when preparing multicolourtransfer sheets to use a range of inks supplied by one manufacturer because in such a range, the vapourisable dyes will have been chosen to be especially compatible, that is to say, they will transfer to a similar extentwhen subjected to similartime and temperaturetreatments in the heattransfer printing process. In general, heat transfer dyes are classified as high, medium or low energy according to the time and temperature conditionswhich are required to causethem to transfer. It is believed that, during the transfer process, the dyes vapourise and reach the surface ofthe substrate to which they are to be transferred (and which must have an affinity for the dyes) and thus the energy classification is to some extent related to the vapour pressure at the transfertemperature. Possible mechanisms for the process of simultaneously embossing and transfer printing as described in United States Patent 4,049,374 may be proposed, based on the above theory of transfer printing.In the case of the invention claimed in Claim 1 of United States Patent 4,049,374 an embossing sheet is used which has no affinityforthe sublimable dyes but which is permeable to the vapour of such dyes during the heat transfer princess.

Typically, the embossing sheet may be spun-laced rayon nonwoven fabric. This embossing sheet is permeabletothedyevapournotonlyduetothe material of which it is made but also to its construction which is sufficiently loose and porous to permitthe ready passage of dye vapour between the filaments of the sheet. In United States Patent 4,049,374 is also mentioned a different process for simultaneously embossing and transfer printing in which the material ofthe embossing sheet is either substantially or largely impermeable to the dyestuff or has an affinity for it so that the vapour ofthe dyestuff reaches the substrate onlythrough defined perforations in the embossing sheet and hence colours the substrate only in the pattern defined by these holes.In this case, the embossing sheet interferes in a defined mannerwith the passage of dye vapourfrom the sheet oftransfer material to the substrate. Lastly, there is claimed in Claim 2 of United States Patent 4,049,374 the use of a vapour permeable embossing sheet into which is incorporated a dye trap in a defined pattern. The dye trap absorbs the dyevapourandso appears as a further pattern superimposed on the pattern in which the dye is present on the sheet oftransfer material.It is not stated in United States Patent 4,049,374 whether the embossing sheetwith the dye trap may be used morethan once without cleaning the dye off the material used as the dye trap but it is readily apparent to anyone skilled in the artthat the material of the dye trap (which may be an acrylic resin) will be soiled by the dye during its trapping action and that it can only be used once as is the case with the embossing sheets described in United States Patent4,049,374which have an affinityforthe dyestuff.

In the process ofthe present invention, a removable plastic film is interposed between the sheet oftransfer material and the substrate. The plastic film may be used eitherto carry a design in material impermeable to vapourisable dye, and so to form a reserved pattern on the substrate, or it may be used to prevent contamination ofthe surface of the substrate by materials present on the sheet oftransfer material.

The mode of action ofthis invention is quite different to that described in United United States Patent 4,049,374 because the plasticfilm is not porous as are the embossing sheets described in the above United States patent. It is apparentthat a simpletestwould easily illustrate this fact. For example, water containing a small amount ofwetting agent (such as a domesticwashing up liquid) would easilyflow through the embossing sheets described in the above United States patent whereas it would not pass at all through the plasticfilm ofthe present invention.

Secondly, the plastic film of the present invention has an affinityforthe sublimable dye and yet it does not require cleaning after each transfer process. Since the plastic film has an affinityforthe sublimable dye, it is coloured bythe dye and clearly a new piece of film will be required each time the decative design is changed.

However, virtually any number oftransfer prints may be made of any particular decorative design using a single piece offilm. This is possible because the sublimable dye (which reaches the surface ofthe plastic film by vapour transfer) passes through the plastic film by moleculardiffusion and will then form vapour which passes to the surface of the substrate by vapourtransfer. That thins is the case is illustrated by the fact that the first two orthree transfer prints made using a relatively thick plastic film are notatfull depth of colour. It is onlywhen the dye concentration has built up through the thickness ofthe plastic film that full depth prints are obtained on the substrate.Of course, to maintain the concentration of dye in the plastic film it is necessarythat a newtransfer sheet is used each time a print is made on the substrate. In this way, a suitable dye concentration gradient is established in the plastic film. Since the dye concentration gradient is established more rapidly in thinnerfilms, it is preferred to usethethinnestplasticfilm possible.

However, the thinnest plastic films are very delicate and the cost per unit area increases as the thickness is reduced be-ofthe increased difficulty of manufacture.

This invention will work with films of virtually any thickness but in order to establish the dye concentration gradient rapidly (that is, after only a few transfers) it is preferable that the plastic film is less than SOp thick. For ease of handling, it is also preferred thatthe plasticfilm is more than 2thick. Even moreto be preferred are plastic films between 2lland 10,thick since these give a good balance of cost and technical properties.In the case where the plasticfilm ofthe invention is used to protect the surface of a substrate from contamination during heattransfer printing, it is preferable that it is as thin as possible and films atthe lower end of the 2pto 1 Op range are recommended. In orderto save handling the unsupported plastic film, it may be releasably adhered to the surface of the substrate, especially where the substrate is a plastic sheet. Afterthe heat transfer printing process has been completed, the plastic film maythen be peeled offthesubstrate and disposed of,togetherwith any contaminating materials.

The process of this invention when used for preparing multiple transfer prints by supporting an impermeable design on the plastic film is ideally suited to the preparation of single colour designs. In this case, the sheet oftransfer material is uniformly coated with a layer of sublimable dye (preferably contained in an ink medium and deposited on the transfer sheet by one ofthe printing processes mentioned above). Multicolourdesigns may be heat transfer printed by the process ofthis invention using multicolourtransfersheets but it is essential thatthe design on the multicolourtransfersheets be registered exactly with the design which becomes imprinted on the plastic film.Ofcourse, this is simple in the case where the multicolour design consists of, say, a few blocks of solid colour than where the multicolour design consists of, say, a four-colour half-tone design. It is also desirable when using the process ofthis invention to print multiple, multicolour designs, to use the thinnest possible plastic film since this limits the amount of 'spreading' of any colour boundaries by sideways diffusion ofthe dye in the plasticfilm. When the process of this invention is usedforcontamination prevention, no registration of multicolour designs is necessary, of course, because each plastic film is used only once.

The composition and properties ofthe plastic film used in this invention are quite important. It is necessary, as has been stated, for the plastic film to possess some affinityforthe sublimable dyes to be used. This may be used as one ofthe criteria for the selection ofthe plastic film since during carrying out the process ofthis invention, the plastic film will become coloured by the dyestuffs used.However, unlike the substrate, the fastness properties ofthe dyed plastic film are unimportant and if a very low rub fastness is apparent, for example, then this does not matter because at the end of the operation, the plastic film will be discarded orstored in some suitable container for later re-use. Avery low rub fastness for the dyed plastic film most likely indicates that whilst the dye molecules can diffuse freely within the film, the affinity ofthe molecules of the dyeforthose of the plastic is relatively weak.It is essential thatthe plastic film is continuous and without any holes or perforations otherwise the dye vapour is able to penetrate to thesubstratethroughthe holes orperforations and so to superimpose on the decorative design an image of the holes or perforations. It is also desirable that the plastic film is uniform in thickness, within the usual commercial tolerances, so that the density ofthe decorative design does not vary according to any varying thickness ofthe plasticfiim. Of course, the thickness ofthe plastic film may be deliberately varied if it is required to achieve a particular effect by so doing. A simple way of carrying the thickness is to use morethan one thickness of plastic film in certain areas ofthe decorative design.However, once the dye concentration has been fully established in the variable thickness film, the effect resulting from the variable thickness will disappear. For this reason, although it is highly desirable thatthe plasticfilm is of uniform thickness exceptwhere special effects may be required, it is not absolutely essential as long as it is accepted that more of the first few few transfers may not be satisfactory. It is essential that the plasticfilm is chemically stable at the transfertemperature. This transfertemperature is dependent, of course, on the substrate employed and on the sublimable dyes used.

A simple means of choosing a plastic film isto use a film composed ofthe same material as the substrate.

In this way, temperature stability, thermal expansion and other compatibility problems are eliminated. In addition, at the conditions which are suitable for heat transfer printing a particular substrate, it is clearthat the diffusion rate ofthe dye molecules in that substrate (and hence in a plasticfilm ofthesame material) is corr ctforthe process of the invention to proceed satisfactorily. As well as chemical stability, it is necessarythatthe plastic film has suitable physical stability at the transfertemperature ie that it does not melt, shrink orwrinkle after one or several transfers.

This is often a matter ofselecting a plastic film grade which has been suitably oriented and stabilised by the manufacturers. The choice can be made by consulting the manufacturers'technical literature on this aspect.

If necessary, the lowest possible transfertemperature combined with an appropriately longertranfertime may be chosen for a particular substrate and plastic film combination to reduce or eliminate shrinkage and wrinkling. Alternatively, the plastic film may be clamped or glued orfixed in some otherwayto a rigid frame to keep it taut and free from shrinkage and wrinkling during the transfer process. Materials of which the plastic film may be composed include acrylic polymers, polyamides, polyesters, copolyesters, epoxy resins, vinyl resins, polycarbonates, polyolefins or combinations ofthese materials. Preferred plastic film materials are polyester and polyamide since these materials are well suited to transfer printing and are often used as substrates.

Polyethylene terephthalate is particularly suitable as a plasticfilm material since it may be obtained in a range of thicknesses, it has an excellent affinity for many sublimable dye molecules and it has an excellent temperature stability.

Where the process ofthis invention is used forthe multipletransferprinting of a decorative design by interposing a dye-impermeable design carried on the plastic film between the transfer sheet and the substrate, it is necessary that the impermeable design is formed in some material which is totally 'opaque' to the sublimable dye. This generally means that it has no affinityforthe dye. Suitable materials are metallic foils such as aluminium or copperfoil or dye impermeable plastics such as polyimides.The impermeable material material must be chemically and physically stable atthe transfertemperature ie it must not shrink or melt, etc. Itis preferable, although not essential, thatthe impermeable design be fixed to the plastic film in some way, such as by adhesion with a suitabletemperaturestable adhesive such as an epoxy or silicone adhesive, etc. If the design is not fixed to the plastic film, it is essential to locatethe decorative design in exactly the same place in relation to the plastic film at each transfer otherwise the dye concentration gradient cannot be established correctly in the plastic film and 'ghost' images ofthe decorative design will be obtained as well asthe desired imprint of the design. A convenient way of forming a decorative design which is attached to the plastic film isto use plasticfilm having a vapour deposited metallic layer and photo-etching away unwanted parts of the metallic layer. If necessary, the metallic layer may be electroplated before or after photo-etching in orderto ensure that it is dye impermeable. Preformed metallic laminates with plastic film may also be be used in the same way with the photo-etching technique. In general, it does not matter which surface of the plasticfilm is used to carry the decorative design. However, if the most accurate definition of the decorative design is required, it is preferable that it is carried on the side ofthe plastic film which isto be in contact with the substrate.In this case, it is necessary that the design formed from the impermeable material is a mirror image of the decorative design which it is required to transfer print on to the substrate. If the decorative design formed from the impermeable material is carried between the transfer sheet and the plastic film, then it must be a right way round image ofthe decorative design. In some cases (for example, ifthe decorative design is composed of adhered metallic foil elements), it is easierto assemble the elements the rightwayround on the surface of the plastic film which will be in contact with the transfer sheet.

The following examples will serve to illustrate the invention. The examples are illustrated by Figures 1 to 3 and in these figures, items are labelled 'A' to 'E'. Item A is the transfer sheetwhith sublimable dye indicated by hatching on the the surface ofthe sheet. Transfer sheetA has a uniform coating of sublimable dye of a single colour. Item B is the plastic film ofthis invention. Item C is the decorative design formed from dye-impermeable material, usually adhered to the plastic film with an adhesive layer which is not shown in the figures. Item D is the substrate on which the decorative design isto be printed bythe heattransfer printing process. Item E is a second transfer sheet on which is imposed a multi-colour pattern in sublimable dyes; the different angles of hatching indicated the different sublimable dye colours.

Example 1 (illustrating multiple transfer printing) In this example, Awas a transfer sheet prepared by screen printingauniformthin layerof MarlerTexTX01 screen printing ink containing a combination of sublimable dyes which give a black colour by transfer printing on to a coated art paper known as Marler Transfer Paper C17. Both the ink and the paper were supplied by E T Marler Ltd. The inkwas printed and dried according to the instructions supplied by the manufacturers. Item B was clear Melinex Type 442 polyethylene terephthalate film manufactured by ICI and its thickness was 23cm. Item C was a cutout shape in 20pthick aluminium foil.This foil was adhered to the face ofthe plastic film B which was to be in contact with the substrate D by the use of Scotch Spray Mount adhesive (manufactured by3M United Kingdom Ltd).

Item D was an off-white pigmented polyethylene terephthalate plastic sheet, MelinexType 226 thickness 250p manufactured by ICI. These items were sandwiched together in the order shown in Figure 1 (ie ABCD) in a heattransfer press (AAdkins Model Double A Series 6000) with the transfer sheet A on top (ieimmediatelynexttothe heated surface ofthe press). The sandwich was then heated for 45 seconds at a temperature of 1900C underthe pressure normally applied bythis press. Atthe end of this time, the sandwich was removed from the press and the items (except B and C) were separated. Item A (the transfer sheet) was discarded.It was then observed that plastic film B was coloured black and that substrate D was uniformly light grey in colour except in the area which had been underthe cutout aluminium foil design. The area which had been underthe cutout foil design was not coloured by the sublimable dye contained on the transfer sheet. The definition of the edges of the design was extremely sharp and there werer no speckles or other marks in any ofthe design areas reserved by the cutout foil design. No shading ofthe coloured areas attributableto structure inthe plastic film B could be detected. Items A and D were then replaced by unused pieces of transfer sheet and substrate respectively and the transfer printing pro cuss was repeated exactly as before using the original items B and C.This time, on examining substrate D, it was observed that the cutout foil design was reserved on a uniform dark grey background. Again, the edges of the design were sharply defined and no speckles or other marks were observed in the reserved areas which had been underthefoil cutout nor was any structure which could have arisen from the plastic film B detectable in the dark grey areas. The printing procedure was then repeated once more with new pieces oftransfer sheet A and substrate D. This time the cutout foil design was fully reserved on a very dark grey background with excellent edge definition and again no detectable structure superposition from the plastic film B.After carrying outthe same transfer printing procedure for the fourth time, again under the same conditions asthose used in thefirst printing,the fully reserved design was present on a black background. The edge definition remained excellent and no structure was detectable which could have arisen from the plastic carrier film B.Afurthersixteen printings were carried out in the same way as the first four and each one gave a reserved image on a black background.Averyslightwrinkling oftheplasticfilm B which had developed at the second printing did not reduce or get any more pronounced bythetwentieth printing and itwasconcludedthatvirtuallyany number of heattransfer printings could be carried out if desired using the same plastic film B and cutoutfoil designCwhichwasadheredtoit. Forcomparison purposes, a sample oftransfersheetAwas heat transfer printed in direct contact with a piece of substrate D for 45 seconds at 190"C. The substrate was coloured black and this colour was indistinguishable from that in the coloured areas ofthe substrate in the above replicate transfer printings4to 20.

Example 2 (comparative example) Example 1 was repeated exactly but item B was in this case Kapton polyimide film Type 30H of 7.5cm thickness manufactured by E I du Pont de Nemours & BR< Co Inc in the USA. In this experiment, even after twenty attempted transfer printing operations, no colour was observed on the substrate D and the plastic film B (iethe Kapton film) was not coloured by the sublimable dye. It was also apparentthatsublimable dye was not being removed from the transfer sheets A because after a successful transfer, the depth of colour on the transfer sheet is reduced and this is an obvious indication thatsublimable dye is being transferred from the transfer sheet.

Example 3 (contamination prevention} In this example (to which Figure 2 is applicable), item A, the transfer sheet, was an electrophotographically prepared transfer sheet on which a design was imprinted in mirror image form in a suitable medium containing a sublimable dye combination which gave a black colourwhen transfer printed onto a plastic or fabricsubstrate. The machinery and materialsfor preparing such electrophotographictransfer sheets are manufactured and sold by Subligraphic SA of Switzerland. In orderforthe electrophotographic process to work satisfactorily, the base material on which the sublimable dye is deposited is a heavily coated electrostatic copying paper. Item B was Hostaphan polyethylene terephthalate film Type RE of thickness 2p manufactured by Hoechst AG of West Germany.Item D was a polyester coated copper satin steel plate (15 guage) supplied by Charterhouse Xpres Ltd ofthe United Kingdom. These items were sand wichedtogetherintheordershown in Figure 2 (ie ABD) and were placed in the heattransfer press described in Example 1. The plastic film B was in contactwith the polyester coated face ofthe polyester coated copper satin steel plate. In this example, the sandwich was placed with the transfer sheet A at the bottom ie the back (non-coated) surface of the copper satin steel plate was immediately nexttothe heated surface ofthe press. The reason for this was that the high heat capacity ofthe copper satin steel would have led to a longer heating time if the heat had to be conducted through A and B.The sandwich was then heated for45 seconds at a temperature of 190"C under the pressure normally applied by the press. At the end ofthis time, the sandwich was removed and transfer sheet A was discarded. A right way round image of the design was present on plastic film B. When plasticfilm B was peeled away from the polyester coating on the copper satin steel plate, this same image W25 present atfull depth ofcolouron the polyester coating ofthe coppersatin steel plate. The imagewas clear and distinct and there was no contamination of any kind on the surface of the polyester coating.

Example 4 (comparative example) Example 3 was repeated exactly except that the plastic film B was omitted from the sandwich. After the heattransfer process, it was observed that the design on the transfer sheet was im printed (the rig htway round) on the polyester coated copper satin steel plate. However, the surface ofthe polyester coating had on it a heavywhite deposit which partly obscured the imprint ofthe design. This white depositwas believed to be a component of the coating on the base material of the transfer sheet. The white deposit could only be removed byvigourous rubbing with a cotton swab soaked in ethyl alcohol.

Example5 (illustrating multi-colour, multiple trans fer printing) Figure 3 is applicable to this example. The transfer sheet E was screen printed with a pattern which consisted of alternate touching bands of ink contain ing yellow and black sublimable dye respectively, each band being 1 Omm wide. All ofthe materials used in the preparation ofthis transfersheet were as in Example 1 with the addition that the ink used for screen printing the bands containing the yellow dye wasMarlerTexTX04. Items B, C and D were exactlyas in Example 1 and the heattransfer process was carried out exactly as in Example 1 except that for each transfer process, the coloured bands oftransfer sheet E were carefully placed in register with those which became imprinted on plasticfilm B.Afterfour transfers, there was observed on substrate D a fully reserved design of the foil cutout C with a background offull depth colour bands of alternate yellow and black.

Example 6 (illustrating alternative position ofthe impermeable design) Example 1 was repeated exactly except that the cutout foil design Cwas positioned between transfer sheet A and plastic film B. The results were the same as observed in Example 1 except that under microscopic examination (at 70 x magnification) the edges ofthe reserved design on substrate D appeared slightly less well defined than those observed in Example 1. However, this difference was not apparentto the naked eye.

Examples 7 - 12 (showing the effect of plastic film thickness) Example 1 was repeated but plastic film Bwas replaced bythe plasticfilmsnoted in the table below.

All other parameters, materials and operations were as in Example 1.

Table 1. The effect ofthe thickness of the plastic film B on the process ofthis invention. Transfer time 45 seconds. Temperature 190 C.

Example c Film Type Thickness Number of Transfers to Full Colour 7 c Type RE 2CI 1 8 Type RE 611 1-2 9 Type RP 12,u 2-3 0 10 1 Type MN 191.1 3-4 11 Maylar D23 231.1 4-5 12 Maylar EB11 50 16 (Hostaphan is manufactured by Hoechst AG: Mylar is manufactured by El du Pont de Nemours & Co Inc.) Example 13 (textile substrate) Example 1 was repeated but in place of Melinex plastic sheet, the substrate, item D, was a warpknitted polyester fabric. In every transfer after the third, a reserved image ofthe foil cutout against a full depth black background was observed on the fabric.

Claims (17)

1. A process of heattransfer printing in which sublimable dye;s tra nsferred to a receiving substrate by molecular diffusion through a removable plastic film having a n affi nit an affinityforthesublimable dye.

2. A process as in 1.above where the plastic film is composed substantially of polyester, copolyester, polyamide, polycarbonate, acrylic, PVC, polypropylene or any combination of these materials.

3. A process as in 1. above where the plasticfilm is composed substantially of polyester, copolyester or polyamide.

4. A process as in 1. above where the plastiofllm is composed substantially of polyethyleneterephtha- late, nylon 6 or nylon 6:6.

5. A process as in 1. above where the plasticfilm is composed substantially of polyethylene terephthalate.

6. A process as in 1. above where the plastic film is composed substantially of nylon 6:6.

7. A process as in 1. to 6. above where the plastic film is between 21l and SOp thick.

8. A process as in 1. to 6. above where the plastic film is between 2p and 251thick.

9. A process as in 1. to 6. above where the plastic film is between 211 and 1 Op thick.

10. A process as in 1. to 6. above where the plastic film is between 2p and 6thick.

11. A process as in 1. to 10. above where the receiving substrate is composed wholly or partially of thermoplastic fibres orfilms.

12. A process as in 1. to 10. above where the receiving substrate is composed wholly or partially of polyester, copolyester or polyamide.

13. A process as in 1. to 10. above wherethe receiving substrate is composed wholly or partially of polyethyleneterephthalate, nylon 6 or nylon 6:6.

14. A process as in 1. to 10. above where the receiving substrate is composed wholly or partially of polyethylene terephthalate or a copolymer of polyethylene terephthalate.

15. A process as in 1. to 10. above where the receiving substrate is composed wholly or partially of polyethylene terephthalate.

16. A process as in 1. to 10. above where the receiving substrate comprises a plastic coating on a support, the plastic coating being composed substantially of polyethyleneterephthalate or a copolymer of polyethylene terephthalate.

17. Heat transfer prints produced bythe process as claimed in claims 1. to 16. above.